1. Field of Invention
The present invention relates generally to packaging of microelectronic devices and more particularly to the packaging of multiple sensors within a thermally conductive vacuum package.
2. Description of the Related Art
A typical, known electronic viewing or sensing apparatus may utilize multiple sensors responsive to various wavelengths or spectrums of propagating wave energy. By viewing an object or scenescape with multiple sensors, the information obtained in a composite image of such object or scenescape derived from the images obtained by all such sensors is advantageously enhanced as compared to the information contained in a single image obtained by a single sensor. The energy wavelengths or spectrums that each sensor is responsive to and the number and combination of said sensors is limitless. The selection of the sensors, and the wavelengths and spectrums to which they are responsive, is determined by the application of the apparatus utilizing such sensors.
For example, a single viewing apparatus may presently utilize one sensor responsive to wavelengths in a first energy spectrum and another sensor responsive to wavelengths in a second energy spectrum. An object or scenescape, when viewed with each sensor, will allow the apparatus to develop an image of the energy emitted in each spectrum from the object or scenescape, and also combine each image into a composite image. Each image as well as the composite image conveys information of the object or scenescape which may then be utilized for further processing by other devices or for human observation.
For the composite image to convey reliable information, the relative alignment of one sensor to the other must be precisely controlled. However, the current state-of-the-art in packaging of such sensors disadvantageously limits the precision by which such sensors utilized by the viewing apparatus may be aligned to each other. This disadvantage and limitation arises from the necessity first to mount the sensor within a sensor package prior to its incorporation within the viewing apparatus.
The need for the sensor package arises from the nature of microelectronic sensors. The sensor is fabricated on a semiconductor chip or substrate, typically silicon or gallium arsenide. An active region of the sensor resides on an exposed surface of the chip. It is the active region upon which the propagating wave energy is incident. The active region may for example be a two dimensional array of active microelectronic devices which are all responsive to the same selected wavelengths or spectrums of wave energy. Each active device develops an electrical signal as a function of the wave energy incident upon it.
The sensor chip is mounted on a motherboard on which it shares common address and data busses with other chips to utilize and process the signals developed by the active region of the sensor. For example each active device within the active region, when addressed, has its electrical signal switched to the data bus, as is well know in the art.
The electrical signals developed by the sensing devices in the active region of the sensor may be temperature dependent or the sensor itself may require active cooling in order for them to operate properly. Therefore, for most sensors, it is important to provide the correct thermal environment, such as temperature stabilization, cooling or thermal isolation, in order for them to operate. For example, when using micro thermal sensors, thermal stability is very important. The thermal stability for these types of sensors needs to be very accurately controlled, otherwise thermal variations in the sensor substrate may cause false signals and images, or a lack of signal to noise ratio will result. For other types of sensors, such as infrared HgCdTe sensors, they must be cooled below ambient temperature in order for the signal to be higher than the noise in the detection, whereas in other types of sensors, such as visible CCD's, the detector needs to be cooled in order to remove the heat generated in the sensor. Also, for the new class of tiny (50.mu..times.50.mu.) detectors to operate, they need to be thermally isolated to eliminate any convective heat transfer and substantially to reduce the conductive heat transfer to the detector element.
Temperature stabilization is accomplished by mounting the motherboard on an obverse surface of a thermal transfer substrate. Mounted to the reverse surface of the thermal transfer substrate are the thermoelectric cooler elements that make up the TE cooler. The combination of the motherboard with the sensor mounted thereon, and the thermal transfer substrate with the motherboard and thermoelectric cooler elements mounted thereto provides a sensor assembly. The sensor assembly is then mounted on a thermally conductive base within the sensor package to conduct heat between the sensor assembly and the ambient environment external of the sensor package.
Thermal isolation is accomplished by mounting the sensor assembly in the sensor package, and then evacuating the internal chamber of the package of gaseous elements. This eliminates all convective heat transfer. Accordingly, the sensor package may also contain at least one getter for removing such gaseous elements from within the internal chamber and maintaining the vacuum.
It is also necessary that the sensor assembly be precisely located within the package. So that wave energy may impinge on the active region, the package also contains a window transparent to the spectrum of energy to which the active region of the sensor is responsive. The package is then mounted on the apparatus utilizing such sensor. For an apparatus that utilizes multiple sensors, the package for each sensor must also be aligned with the package for each other sensor.
To accomplish such alignment, the sensor package is provided with alignment indicia, also referred to as datum. The same indicia are typically used for alignment of the sensor assembly within the package as well as referencing alignment of the package to other packages. It is known that tolerance errors will exist for the alignment of each sensor assembly to its respective package and that further tolerance errors will exist for the alignment of each package to another package. All of these tolerance errors will be additive into an overall alignment offset error for each sensor to another sensor. For example in an apparatus utilizing two sensors, the alignment offset error of the sensors to each other will be determined by the adding the tolerance error of each sensor assembly to its respective package, and the tolerance error of the packages to each other. Of course, these tolerance errors may have both positive and negative values. Statistically, the range of overall alignment offset error for all of the devices built in a manufacturing run should demonstrate a Gaussian, or normal, distribution. Those apparatus having an overall offset error above or below a threshold may be unacceptable. It is in these potentially unacceptable devices where the individual tolerance errors were all primarily negative or positive.
It would therefore be highly advantageous to provide a novel sensor package that reduces the range of overall offset error occurring in a manufacturing run. It would also be highly advantageous to provide a sensor package that provides for the alignment of multiple sensor assemblies in a single package.